1. Field of the Invention
Aspects of the present disclosure relate to electrolytes for lithium secondary batteries and lithium secondary batteries including the same, and more particularly, to electrolytes for lithium secondary batteries which improve the high temperature lifetime characteristics and high temperature conservation characteristics of the batteries.
2. Description of the Related Art
Lithium secondary batteries are rechargeable at a high speed, and have an energy density per unit weight that is at least three times larger than that of existing lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, or nickel-zinc batteries, and thus research on developing lithium secondary batteries is actively conducted. Generally, a lithium secondary battery includes a positive electrode, a negative electrode, and a separator and an electrolyte disposed between the positive electrode and the negative electrode. With respect to electrolytes, methods of improving the performance of lithium secondary batteries by adding small amounts of additives to the electrolyte without affecting the properties of the electrolyte are highly desirable.
Electrolyte additives have various functions, and some additives serve to form a film, for example, a solid electrolyte interface (SEI), to prevent direct contact between electrode active materials and electrolytes. The additives that form a film on the electrode surface are classified into negative electrode additives helping to form an SEI on a graphite surface and over charge protection (OCP) additives helping to form a thick film on a surface of a positive electrode.
Recently, research on high voltage positive active materials has actively implemented an increased demand for lithium secondary batteries with high energy densities, e.g., a battery for an electric vehicle. However, research has not yet been successful in producing electrolyte additives for preventing the oxidization of electrolytes occurring on surfaces of positive active materials.
Generally, the potential window of an electrolyte should be wider than the potential difference between positive and negative active materials. However, since active materials for a high voltage are used for increasing the energy density of a battery, the potential window of an electrolyte becomes narrower than that of the electrode active materials. Therefore, a film that prevents direct contact between electrolytes and electrode active materials should be formed to prevent electrolytes from decomposing.
The use of aromatic compounds such as biphenyl and terphenyl as electrolyte additives forms a thick film on the surface of the positive electrode when the voltage of the battery becomes sufficiently large, and thus the Li ions cannot pass through the film, thereby stopping current flow. That is, such additives serve to prevent overcharge. Recently, a method of forming a thin film on the surface of a positive electrode using such additives with a low concentration has been proposed. However, it is difficult to obtain satisfactory battery characteristics, thus leaving much room for improvement.